API: get curation data for a set of genes

[ajo2995]

Given a list of gene ids, fetch the opinions and reasons with their frequency from the database. The format can be a list with one entry per gene:
[ { "id":"gene id", "opinion": "okay/flag", "reason": "none/5prime/3prime/...", "count": number of people with this opinion and reason} ]

This denormalized format can be easily aggregated in different ways by the client.